# BLIGH’S CREEP THEORY

• This theory is base on the assumption that seeping water through the soil below the weir
• Follows the path along with the contact of the base, with the underlying sub-soil.
• The length of the path of seeping water from the point of entry into the sub-soil from the U/S of the impervious.
• Apron to the point at the D/S end of the impervious apron is know creep length.
• Bligh also assumed that loss of head of the seeping water is proportional to the length of its travel irrespective of whether.
• The length of travel is in the horizontal or vertical direction.
• He also assumes that unless cut-off sheet piles extent to the impervious subsoil strata, no amount of sheet piling could stop the flow of percolating water.
• AB is the length of impervious apron l and H is the head of water-fill up to the top of the weir CD and there is no water on the D/S side.
• According to Bligh’s theory L = l where L is the total creep length.
• If vertical cut-offs are provide below the impervious apron.

$L= 1+2d_{1}+2d_{2}$

• The Length of vertical cut-off  taken double because vertical cut-off provides the creep length equivalent.
• To twice the length of the cut-off, as seeping water once goes down and then comes up along the cut-off.
• If H is the total head causing seepage or total loss of head, and L the total creep length, the loss of head per unit length of creep (C) is given by

$C= \frac{H}{L}= \frac{H}{1+2d_{1}+2d_{2}}$

• Loss of head per unit length of creep (C) is know  percolation coefficient.
• The reciprocal of percolation coefficient is know the coefficient of creep (C).
• Safe values of coefficient of creep for different soils are given below.

Weir design by Bligh’s theory.

• According to Bligh’s theory, two design criteria are to be consider.

(i) Safety against undermining or piping.

• To safeguard the weirs against failure by piping, the creep length should be provid according to the following formula:

L = CH, where C is coefficient of creep.

• Such a length would provide safe hydraulic gradient and seeping water emitting from D/S end of the impervious apron.
• Will not be having sufficient uplift pressure to dislodge the soil particles.
• This will avoid boiling action of soil at the D/S end of the apron.

(ii) Safety against uplift pressure.

• To counter balance the force of uplift, sufficient floor thickness should be provid, specially on the D/S side of the weir.
• Let at distance L1 creep length from U/S end of the impervious apron h is the resultant uplift head, the net uplift pressure wh can be comput follows